Conventionally there is known, as a switch for use on an electric circuit, a switch using an air bridge described in U.S. Pat. No. 6,218,911. In this structure, a movable air bridge is arranged between a pair of electrodes formed on a substrate. In case an electrostatic force is given between the electrode and the movable air bridge, the air bridge horizontally moves toward the electrode into a contact with one electrode but isolated from the other electrode. Accordingly, in case a signal is inputted to the air bridge, the air bridge is electrically connected with the one electrode, allowing a signal to pass. However, the signal is cut off at the other electrode, thus enabling switch operation.
Meanwhile, a micro-electromechanical RF switch is known which is described in U.S. Pat. No. 6,307,452. The micro-electromechanical RF switch has a plurality of folded spring suspension devices on a substrate, on which a micro-platform is suspended. Beneath the micro-platform, a signal line is formed. When a direct current potential is applied between the signal line and the micro-platform, an electrostatic force is caused to attract the micro-platform toward the signal line, thus effecting switch-on.
However, in the structure of U.S. Pat. No. 6,218,911, in the case of driving the air bridge on an electrostatic force, realizing greater signal isolation requires to increase the spacing between the electrode and the air bridge. However, because electrostatic force is proportional to a negative square of distance, electrostatic force decreases and makes it impossible for response time to attain a desired value. Meanwhile, there is an approach to increase the application voltage in order to compensate for the decrease of electrostatic force. However, application voltage increase is not preferred for the radio communication device requiring low power consumption and low drive voltage.
Meanwhile, because the air bridge is of a straight-beam structure, tensile stress if exists within the beam increases the rigidity against electrostatic force just like a strongly stretched cord, raising a pull-in voltage (pull-in voltage due to electrostatic force). Furthermore, at an elevated temperature, beam internal stress turns into compression, possibly causing buckling. Namely, unless the residual stress resulting from a manufacture process or environmental temperature upon switch operation can be controlled constant, stable switch operation characteristic cannot be guaranteed.
On the other hand, the micro-platform structure in U.S. Pat. No. 6,307,452 is divided with a region for coupling to a signal line and a folded spring-suspension structure part (flexure) for relaxing stress. Namely, an additional structure is provided to relax internal stress. As apparent from Newton's laws of motion, in the case of applying the same force to a structure having a mass m, the acceleration occurring on the structure is greater as the mass m is smaller. For this reason, the above structure involves the problem that, because of addition of the flexure, the mass m is increased to make it impossible to increase the response speed. Meanwhile, as the flexure is softer, the platform is relaxed in binding at its supports. Consequently, in case there exists a stress gradient in a direction of film thickness, the platform warps up due to stress release and separates off the substrate. Unless the stress gradient value cannot be accurately reproduced in the beam manufacture process, the degree of warpage varies, making it impossible to suppress the variation in capacitance reduction between a platform and a signal line and the variation in pull-in voltage increase. Meanwhile, the manufacture with using a semiconductor process makes a beam and a flexure structure into the same material of conductors. In a radio frequency circuit, the flexure part thereof has an non-negligible impedance.
Meanwhile, where the environmental temperature changes, thermal stress takes place due to a difference of thermal expansion coefficient between the base material and the beam material. Although the thermal stress is different in occurrence cause from the foregoing residual stress encountered in manufacture process, it triggers a phenomenon of the similar “strain in the beam due to stress release”. Accordingly, it must be taken into account of an effect upon capacitance or pull-in voltage.